Mobs are not dropping their heads

I have been playing the windows 10 version of Minecraft for about 6 months now and have yet to have any mob head drop after they have been killed. This is true for hostile and passive mobs. True for naturally spawning mobs (which is another issue of mine) and from spwaners. I only play difficult in Survival. My sword is enchanted to looting 3 and still no head drops.

Although I am new to Minecraft I feel as though within a 6 month period of time I would have had at least one head drop. I'm trying to make a beacon but am unable to because of this issue.